Sle1 is the strongest susceptibility locus in the NZM2410 mouse model of SLE. We have shown that it mediates the loss of tolerance to nuclear antigens, and that its expression is required for full reconstitution of the disease. In the process of developing a high resolution map of Sle 1, we have demonstrated that it corresponds to three loci, Sle1a, Sle1b, and Sle1 c. Each of these loci results in autoantibodies against nuclear antigens, but in a different profile of lymphocyte marker expression, suggesting that they belong to different functional pathways. In parallel with gene identification efforts being conducted on these genes, we propose to characterize how these loci contribute to SLE pathogenesis, either by themselves, in combinations with each other, or in combination with other SLE-susceptibility genes. This study is necessary because, in most cases, gene identification does not provide information on the mechanisms by which this given gene is involved in the disease process. In addition, analyses of polygenic autoimmune diseases have clearly shown that pathogenesis results from interactions between weak loci. We have developed during the initial funding of this proposal a unique model to dissect out these interactions based on gain of phenotype analyses. Our model now comprises a large collection of congenic strains that either contain small genomic intervals with a single susceptibility locus, or various combinations of them. We have also compiled a large database of tissues and phenotypes corresponding to these strains. This proposal has four specific aims: 1) To delineate the impact of Sle1a, Sle1b, and Sle1c on lymphocyte functions and to dissect the interactions between these loci. These experiments will combine characterization of B and T cell development associated with each locus, breeding of the uMT and Tcra null mutations to assess the impact of the Sle1 loci on intrinsic B and T cells defects, and bone marrow chimeras to analyze interactions; 2) To evaluate genetically and functionally Cr2 as candidate gene for Sle1c. We will generate recombinants to assess whether Sle1c co-segregate with Cr2, produce BM chimeras to localize the functional expression of Sle1c between B cells and FDC, and evaluate the role of Sle1c in anti-dsDNA tolerance by using the 3H9 sd-tg system; 3) To assess how Sle1a, Sle1b, and Sle1c interact with other SLE loci, using Ipr, Yaa, and Sles1 as models. We have shown that Sle1 co-expression with either Yaa or lpr results into a highly penetrant SLE, while Sles1 shuts down Sle1 phenotypes, and that each Sle1 locus engages into a different type of interaction with these loci. We propose to use this system to dissect the role and mechanism of interactions in SLE pathogenesis, using immunological characterization gene expression profiling, and a bone marrow chimera approach to trace the fate of B cells expressing a pathogenic combination of loci.; 4) To characterize two new susceptibility loci on chromosomes 10 and 11 and their interaction with Sle1. Two new susceptibility loci have been identified and we propose to combine them with Sle1 to characterize their contribution to the disease and to map their location. These experiments are part of a multi team effort to identity and characterize the genes responsible for a major SLE-susceptibility locus in order to be able to design therapeutic interventions on these genes and their functional pathways.